That's very interesting about your Epic.
I'm getting my fork through a local guy who sells a lot of Fox forks. He called them up to find out more about the SC since he hadn't sold one of those yet. The Fox Factory guy told him that all the high end Treks are specifically 51mm because of the geometry. He said that a lot of it has to do with head tube angle, but other geometry plays a factor in contribution, like wheelbase, chain stays, etc...
I'm from the old school days and I remember when rigid forks on most bikes were bent where the rake was more obvious. and if you ever did silly stuff like try and turn the handle bar around backwards and ride no-handed, you were in for a quick surprise kiss with the pavement as the rake prevented the ability for the wheel to track. the physics are similar to dihedral in an airplane wing. Stunt planes have little to no degree of dihedral to prevent them from trying to roll back over when inverted, where as jumbo jets have much greater angle. That force is is similar to pressure applied from the weight of the rider. The rake has kicked the axle positive of the centerline of the head tube vector. Therefore Delta (D), being the direct straight line from axle to bottom headset crown (connecting pivot point) which creates a weighted mass between the 2 angles, (similar to the center of gravity in a dihedral wing) applying force to F (gravity).
In an airplane, it's obvious to observe and understand, the wings are equal length and F is down (gravity). In a bike, force from the rider is down, but the force (F) applied to the rake if forward. The 'dihedral' type concept is harder to observe because the angle is offset and between the head tube vector and rake vector (it looks lopsided). However, if you hold your bike up by the top tube in a wheelie where the head tube is parallel to the ground, what happens to your front tire? It tries to turn around backwards. The more rake offset, the greater the force an faster it will come around once it breaks 0 degrees, which is similar in principle of greater angle of dihedral rolling a plane back upright.
[Sorry about that long explanation, I program games and have to do a bit of physics and math simulations from time to time.]
I remember this in action: The guy I worked for in his bike shop back in the 80's would weld up custom kits, and we got to play with frame geometry and experiment a little... (and burned up all kinds of stuff around the shop with his torch and welder). I do remember we built a custom fork for my Haro Master BMX bike. I was getting into riding half pipe more than street freestyle and the master was super twitchy compared to the other model they made (forgot the name). I believe what he did was increase the rake on the fork he built for me. It made the bike track better when riding with no hands, and definitely took some of the twitch out on landings, I was not the most graceful half pipe rider.
